Method and composition of preparing lead alkyl compounds



METHOD AND COMPOSITION OF PREPARING LEAD ALKYL COMPOUNDS Sol B. Wiczer, 1625 Eye St. NW., Washington, D.C.

No Drawing. Filed Nov. 1, 1952, Ser. No. 318,310

6 Claims. (Cl. 260-437) This invention relates to preparation of lower lead alkyl compounds particularly tetra alkyl lead compounds having 1 to 3 carbon atom alkyls usually ethyl or methyl lead alkyls, and mixed lower lead alkyl compounds.

While it has been generally suggested in the art that lead alkyl compounds may be formed by reaction of a lead alloy with an alkali metal together with an alkyl halide in forming tetraalkyl lead compounds, the method conventionally practiced in the art has always been so dium alloys with lead and while the sodium lead alloy has been suggested to range in proportions of equal molecular quantities of sodium and lead up to 2 to 2 /2 times the molecular proportion of sodium to lead, the yields obtainable in practice have been notoriously low, usually between and yields based upon the quantity of lead present in the reaction.

Proposals have also been made in the art to prepare tetra alkyl lead compounds by the Grignard reaction, that is, the reaction of lead salts with alkyl magnesium halides, or alkyl lithium halides, but such reactions while continuously reported in the literature have not been used commercially to prepare lead alkyl compounds on a commercial scale for reasons inherent to this type of reaction.

There are many incidental problems in the carrying out of the reaction to form tetra alkyl compounds by the reaction of sodium lead alloys upon alkyl halides. The reaction is slow and dangerous and despite the same, low yields only are always obtainable rendering the reaction quite expensive.

According to the present invention, I have found that metallic sodium may be replaced in whole or in part with metallic lithium to carry out this reaction under better reaction conditions which run more smoothly at lower temperatures and to good yields, whereby this reaction is a substantial improvement upon the common sodium lead alloy reaction method. In addition to producing good yields, the use of the lithium lead alloy, according to the present invention, has certain additional advantages over the sodium lead alloys of the prior art. The lithium alloy will react over a much Wider temperature range, from about minus 40 C. up to any higher temperatures as are most convenient for commercial production, for example, up to 90 C. The reaction runs smoothly at lower temperatures and gives good yields, hence such are preferred, for example temperatures ranging from minus 40 to plus C. Such temperature limits allow accommodation to commercial procedures and less expensive apparatus. Thus for example, where, as proposed in US Patent 2,597,754, a lead sodium alloy may be ground in ethyl chloride to reduce the lumps of sodium lead alloy to a preferred particle size range of 8 to 20 mesh, the ethyl chloride being liquid at temperatures below about 12 C. to provide a liquid grinding medium for size reduction of the lumps of lead sodium alloy, I have found that when a lead lithium alloy is so comminuted, the reaction with the alkyl halide will actually take place during the grinding mixing procedures with the alloy to form the lead alkyl compound at these low temperatures.

It will be apparent with such procedure therefore that the lead alkyl compound can be produced in the absence 2,960,515 Patented Nov. 15, 1960 ice of solvents and use of substantial pressures which require expensive apparatus construction for moving catalyst and reagents across pressure differentials, and to carefully control seals for maintaining such very difliculty handleable and highly toxic lead alkyl compounds. For example, in the ordinary sodium lead alloy reaction, where the product is reacted at high temperatures up to C. and substantially high pressures are developed at this temperature by extraneously added solvent and highly volatile alkyl halide, careful processing control of this normally dangerous reaction in rugged apparatus is necessary. According to my proposed procedure, high pressures are unnecessary and the apparatus used therewith does not need to be heavy and strong to resist substantial pressures. Moreover the reaction can be carried out without solvent which simplifies the method in the manner of removal thereof.

Thus according to the preferred procedure the reaction may be carried out at a low temperature with liquid or refluxing alkyl halide and at the same time that the lead lithium alloy is being size reduced to optimum particle size for reaction. The reaction may be carried out by continuous feed of reagents and size reduction in a continuous or semi-continuous manner thereby greatly reducing the production costs of the reaction as well as greatly improving safety factors for this dangerous reaction, allowing the reaction to run at a controlled rate with simple apparatus and with comparable or higher yield to efiect production of lead alloys more economically.

The lithium metal as small pieces, usually chopped lengths of lithium wire of V to /2 inch size preferably about /s to A inch, are mixed with lead chips or gran ules of 5 to 25 mesh and the mixture placed in a sealed vessel such as a stainless steel bomb which is swept clear of oxidizing gases with substantially pure nitrogen or other inert gases such as helium and heated to a fusion temperature between 300 and 500 C. The ratio of lithium may be from 1 to 4 mols of lithium per mol of lead usually about 1 to 2.5 mols of lithium per mol of lead. Where a mixed lithium sodium alloy is used the total alkali metal will be in the range of 1 to 4 mols per mol of lead, but lithium used in such mixture only for purposes of activation may be reduced to about 2 mols, the balance being sodium to a total of 1 to 4 mols of mixed alkali metals. The cooled dry alloy is placed in a mill such as a ball mill which is swept with hydrocarbon gas and milled with steel balls, or otherwise crushed to small granules of the order of S to 25 mesh usually 8 to 20. The lead lithium alloy thus described is then reacted with the ethyl chloride preferably at low temperatures without a solvent but a solvent may be used if desired for commercial production. Where high pressure apparatus is available the reaction will also run at temperatures as high as 90 C. and under pressure, but this method is not preferred.

Example I 221 grams of finely divided, about 10 mesh particles of lithium lead alloy comprising one mol of lead alloyed with two mols of lithium are placed in a round bottom flask fitted with a refluxing condenser both surrounded by a cooling bath containing an ice-salt mixture. The flask has also a dropping funnel for ethyl chloride and a gas inlet tube for nitrogen and a slow agitator. Nitrogen gas is allowedto continuously enter the reaction vessel slowly and to sweepout the air for a period of a few minutes prior to starting the reaction. 129 grams of liquid ethyl chloride at a temperature of -20 is placed in the dropping funnel which is also surrounded by a freezing mixture comprising ordinary ice and solid calcium chloride chips to maintain the temperature thereof at appromixately -20. The ethyl chloride is added slowly dropwise and falls dropwise upon the lead alloy which reacts immediately, the volatilized ethyl chloride being returned from the refluxing condenser. After about 10 minutes of addition the lead alloy becomes wet with reaction product and agitation is slowly begun to efiect stirring of the wet Slurry. Addition of the ethyl chloride is completed over a period of about an hour, the addition being regulated by the rate of the reflux of ethyl chloride appearing at the condenser. After addition of the ethyl chloride the reaction mixture is then allowed to stand at the refluxing temperature until refluxing substantially ceases. The cooling medium is then withdrawn from the reaction flask which is allowed to stand with slow agitation for an additional period of about one hour until the temperature rises to approximately room temperature. The lead tetra ethyl formed may be taken up in ether or distilled directly under vacuum from the reaction vessel. 147 grams of lead tetra ethyl is recovered by vacuum distillation. In a modification of this example, the lithium lead alloy in chips of larger particle size than that ultimately desired such as about mesh, are placed in a reaction vessel equipped with a slowly rotating stirrer and steel balls are simultaneously placed therein and the stirring operation is maintained continuously to eflect a size comminution of the lead lithium alloy while the ethyl chloride is slowly being added in a manner as described. Thus the particles of lead lithium alloy are continuously being broken up and stirred as the ethyl chloride is being added thereto.

Example 2 A lead lithium alloy containing 3 mols of lithium per mol of lead are reacted in quantity of 228 grams of alloy with 194 grams of liquid ethyl chloride added dropwise thereto at a refluxing temperature of about 12 C. The addition is efiected over a period of 100 minutes, a rate continuously regulated by quantity of reflux and the reaction mixture allowed to stand with stirring until the reflux ceases while maintaining the temperature thereof. Thereafter the temperature is allowed to rise to room temperature and the lead tetra ethyl is vacuum distilled. 160 grams of lead tetra ethyl are obtained.

Example 3 100 grams of lead lithium alloy containing two mols of lithium per mol of lead ground to a particle size of about mesh are suspended in 200 cc. of dry hexane and chilled to -40 C. and placed in an autoclave. Thereafter 115 grams of liquid ethyl chloride at about 20 C. are added to the liquid hexane. The container is swept out first with nitrogen, sealed and the temperature allowed to rise to room temperature and is then heated to 70 C. for 20 minutes. The yield was 63 grams of lead tetra ethyl obtained after venting the excess ethyl chloride into the atmosphere and removing the solvent and distilling the lead tetra ethyl in a vacuum.

Example 4 A lead-lithium-sodium alloy comprising 1 mol of sodium, 2 mols of lithium, and 1 mol of lead formed by fusing together a divided mixture to form the alloy are reacted in proportion of 122 grams of the alloy in particle size of about 10 mesh with 120 parts of ethyl chloride by placing the alloy in 200 parts of normal hexane, cooling the same to 0 C. then adding the 120 parts by weight of liquid ethyl chloride. The reaction mixture is placed in an autoclave, allowed to warm slowly to room temperature for a period of 30 minutes and is then heated to 85 C. The yield is 48 grams of lead tetra ethyl after removing the solvent and distilling at reduced pressure.

As thus described, lead alkyl compounds are formed by reacting an alkyl halide with lithium lead alloy in substantially high yields and over a wider range of temperature even reacting at extremely low temperatures at a desirable rate.

Certain modifications will occur to those skilled in the art. For preparation of lead alkyls or mixed lead alkyls as desired, the corresponding alkyl halide will be used. Any alkyl halide including bromides and iodides may be substituted for the chloride in these examples. Similarly when mixed lead alkyls are desired then mixed alkyl halides will be added for reaction with the lead lithium alloy. Similarly di-lead compounds may be prepared by this procedure to form the type of compound disclosed in my prior US. Patent 2,447,926 by substitution of a proportionate quantity of alkylene di-halide for some of the alkyl halide to form such compounds in a manner which will be evident from the disclosure in this patent. Certain activators may be added to the reaction mixture as known in the art to accelerate the reaction such as active ketones and aldehydes. Similarly, since the lithium lead alloy is substantially more active than the sodium lead alloy, for purposes of economy the lithium may be made up as a combined alloy with the sodium merely to activate the latter.

I claim:

1. The method of forming tetra lower alkyl lead com pounds comprising reacting a lower alkyl halide with a lithium lead alloy in a volatile solvent selected from the group consisting of the said reaction lower alkyl halide in liquifled state and volatile hydro-carbon at a temperature at least in the initial phase of the reaction, below about ambient temperature and at about atmospheric pressure.

2. The method as defined in claim 1 wherein the reaction is effected with a mixed alloy of lithium sodium and lead.

3. The method as defined in claim 1 wherein at least the initial phase of the reaction is carried out at a temperature in the range of -40 to about 20 C.

4. The method as defined in claim 1 wherein the lead lithium alloy is an alloy of l to 4 mols of lithium per mol of lead.

5. The method as defined in claim 1 wherein the lead lithium alloy is a mixed alloy of lithium and sodium with lead in the approximate ratio of 1 mol of lead to l to 4 mols of both lithium and sodium, the lithium being present in proportion of at least about 0.2 mol per mol of lead.

6. The method of producing a tetra lower alkyl lead compound comprising reacting a liquified mono lower alkyl chloride normally volatile at temperatures above about ambient temperature, at approximately the temperature of vaporization of the said liquified alk-yl halide with lithium lead alloy, said liquified alkyl halide acting as the sole solvent medium for the reaction.

References Cited in the file of this patent UNITED STATES PATENTS 1,360,348 Worrall Nov. 30, 1920 1,652,077 Welter Dec. 6, 1927 1,652,078 Welter Dec. 6, 1927 2,167,828 Dowdell Aug. 1, 1939 2,535,191 Calingaert et al Dec. 26, 1950 2,591,509 Calingaert et al Apr. 1, 1952 2,621,199 Gilbert Dec. 9, 1952 2,621,200 Kolka et al Dec. 9, 1952 OTHER REFERENCES Chemical Reviews, vol. 54, No. 1, February 1954, page 110. 

1. THE METHOD OF FORMING THE TETRA LOWER ALKYL LEAD COMPOUNDS COMPRISING REACTING A LOWER ALKYL HALIDE WITH A LITHIUM LEAD ALLOY IN A VOLATILE SOLVENT SELECTED FROM THE GROUP CONSISTING OF THE SAID REACTION LOWER ALKYL HALIDE IN LIQUIFIED STATE AND VOLATILE HYDRO-CARBON AT A TEMPERATURE AT LEAST IN THE INITIAL PHASE OF THE REACTION, BELOW ABOUT AMBIENT TEMPERATURE AND AT ABOUT ATMOSPHERIC PRESSURE. 